There is no "official" method of converting a string into a WEP key. Each wifi vendor has created their own algorithm to do it. The hex number is the only important/useful one. The string to hex feature is just a convenience they throw in.
So, based on that, there's really no way of determining the algorithm based on one sample of the conversion. With several sets of input/output it may be possible, but it would mostly be just trial & error.